The present invention relates to a method for marking an insertable portion of an endoscope, and more particularly, a marking method for forming marks on an outside surface of an insertable portion of an endoscope.
As is well known, an endoscope may be used to observe the inside of a non-accessible area. For example, as shown in FIG. 2, when a foreign substance 15 is located within a duct 14 of a small diameter (decreasing the efficiency of duct 14), an insertable portion 12 of an industrial endoscope is inserted into the duct 14 to observe its inside. An objective optical system is provided on a distal end portion 13 of the endoscope to observe object 15. When any foreign substance such as substance 15 is found, an approximate range to the foreign substance 15 can be measured by knowing an inserted length l of the insertable portion 12. Thus, marks 16 (range scales) are provided over the whole length of the insertable portion 12.
Endoscopes for the use as described above are employed in numerous environments. Accordingly, an outer cover of the insertable portion of an endoscope must have a high chemical and mechanical durability. Similarly, marks 16 formed on the outer cover of the insertable portion must have a high durability so as not to fade due to chemical or mechanical action.
Conventionally, marks have been formed as disclosed in, for example, Japanese Patent Publication Sho No. 57-22575 and Japanese Utility Model Publication Sho No. 59-42002, in such a way that marks are drawn on the outer surface of an outer cover of the insertable portion of an endoscope with a marking material and are heated to deposit them thereon. In this case, the outer cover is made of ethylene tetrafluoride resin and the marking material is a synthetic ethylene fluoride resin having a lower melting point than that of the outer cover and includes a desired pigment.
However, the sheath tube of the insertable portion of an endoscope does not include the fine braided metal wire. Rather, the ethylene hexafluoride resin is merely applied to a ethylene tetrafluoride resin tube and baked at a temperature of 200.degree. to 800.degree. C. to deposit the marking material thereon. Marks on endoscopes produced according to this method do not have a high mechanical durability.
Furthermore, in the conventional marking method, as described above, the sheath tube comprising the insertable portion of the endoscope is made of ethylene tetrafluoride resin, and the marks are drawn on the outer surface of the outer sheath tube with ethylene tetrafluoride resin mixed with a pigment of a desired color. The tube with the markings thereon is baked at a temperature of 200.degree. to 800.degree. C. to adhere the markings to the tube.
However, problems of durability persist. The ethylene tetrafluoride resin is susceptible to being shaved off or damaged, particularly in industrial applications. Consequently, it is impossible to assure the durability of marks formed from ethylene hexafluoride resin on an endoscope surface.
In order to obtain a sufficient degree of mechanical durability, an outer sheath tube 17 for an endoscope is proposed in which fine metal wires 18 (such as stainless steel wires) are braided on a plane, as shown in FIG. 3. In order to prevent release (flying out) of cut fine metal wires and to give hermetic sealing of an outer cover, an outer sheath tube 17 for an endoscope is developed which is formed by impregnating fluorine-containing rubber (fluorine elastomer) 19 between wires and then vulcanizing it. In order to provide marks 16 which are distinguishable, different colors are formed on the outer cover, however, adhesiveness of a marking material to the fine metal wires and the impregnated fluorine-contained rubber is required. In fact, even when a commonly used coating varnish is dissolved in a commonly used solvent and is applied to the outer cover, it has a poor adhesiveness and easily comes off in an environment of 20.degree. C. which is common for an industrial endoscope.